Nacreous or pearlescent pigments exist in nature in the form of plate-like guanine microcrystals from fish. Because of high cost, source variations and unsuitability for some applications, a number of synthetic nacreous pigments have been developed.
One the most important synthetic nacreous pigments is titanium dioxide-coated mica which is composed of a mica platelet having an adherent crystalline titanium dioxide coating thereon. The color exhibited is a function of the thickness of the coating. The composite pigment has good reflectivity characteristics, high stability to heat and chemical agents and is non-toxic, which makes it suitable for cosmetic application. The titanium dioxide may be in the anatase form as described, e.g., in U.S. Pat. Nos. 3,087,827; 3,087,828; 3,418,146; and, 3,437,515, or in the rutile crystalline form as described in U.S. Pat. No. 4,038,099.
Metal oxides other than titanium dioxide have been used to prepare similar types of nacreous pigments by coating on the mica platelets. Some examples include ferric oxide (U.S. Pat. No. 3,087,829), zirconium dioxide (U.S. Pat. No. 3,087,828), tin dioxide (U.S. Pat. No. 4,040,859) and the like.
The metal oxide coating on the mica substrate has a high refractive index and provides the optical effects, including high luster or reflectivity, coverage, interference reflection color (if the metal oxide coating is sufficiently thick) and absorption color (if the metal oxide contains color material). The mica, on the other hand, has a low refractive index and essentially functions solely as a carrier or substrate, making almost no contribution to the optical effect of the pigment. However, the weight of the mica in such a pigment is significant, usually amounting to about 40%-90% and most usually in the range of 60%-80% of the entire weight.
Despite preparatory procedures, variations from the natural source of the mica persist into the final product. Most sources are unsuitable for use in nacreous pigments because they are quite dark or cannot be adequately ground to yield the desired dimensions for the pigment platelet substrates. In addition, mica darkens considerably during the calcination process used to prepare the metal oxide-coated mica product and this results in undesirable color effects. The weight contributed by the mica substrate can also be a problem in some applications because the maximum loading or concentration of the pigment is reached before an optimum in optical properties of the system is obtained.
Unsupported crystalline titanium dioxide platelet nacreous pigments are described in U.S. Pat. No. 3,861,946. The unsupported pigment is realized by coating the titanium dioxide on calcium sulfate chips and platelets followed by dissolving away the calcium sulfate substrate.
U.S. Pat. No. 4,192,691 describes the preparation of unsupported metal oxide nacreous pigments by treating a metal oxide-coated mica nacreous pigment with an aqueous acid solution containing hydrofluoric acid and mineral acid until the desired amount of mica has been extracted. The resulting pigment can be used in a wide variety of cosmetic and plastic applications and possesses improved stability, luster, mechanical integrity and other properties.
A number of the patents describe the removal of the substrate from metal oxide-coated substrates and particularly the dark mica varieties such as phlogopite and biotite. U.S. Pat. No. 4,883,539 describes a process in which a metal oxide-coated phlogopite mica pigment is treated with a mineral acid such as sulfuric acid, hydrochloric acid or nitric acid, preferably combined with some oxidizer such as nitric acid or hydrogen peroxide. As pointed out in U.S. Pat. No. 5,076,849, this results in a product which can be too heavy for certain purposes, can have too weak interference colors for producing the desired nacreous effect and it is difficult to grind the pigment particles into a suitable nacreous pigment size. The last named patent, therefore, seeks to improve the process by conducting a two step process in which the acidic extraction is followed by extracting the acid treated pigment with a base. This procedure is preferably applied to dark mica sorts such as phlogopite or biotite which have lower aluminum content and are therefore easier to dissolve rather than light mica sorts such as muscovite which do not favor a selective extraction of cations. A major problem with this two-step procedure, however, is that it generates an enormous amount of waste products, especially when muscovite mica is employed. For instance, approximately 20 parts of waste are generated for each part of final product produced. The necessity of disposing of such a large quantity of waste, together with environmental concerns, makes this process impractical.
It will be appreciated from the foregoing that there remains a need for a more efficient process of removing mica from metal oxide coated mica nacreous pigments. It is accordingly the object of the present invention to provide such a process.